Linear Polarization Observations of Water Masers in W51 M

High-resolution polarization observations of water masers provide a powerful tool for studying Alfvenic turbulence and magnetic fields in dense circumstellar regions. Here we present some main results of the first 22 GHz linear polarization observations of water masers in the central low-velocity range of W51M, 54 < Vlsr < 68 km s−1, obtained with VLBA (Leppänen, Liljeström, & Diamond 1998). The principal difference of polarimetric VLBI from total intensity VLBI is the need to calibrate the instrumental polarization parameters, which have been solved by Leppänen (1995) with a feed self-calibration algorithm (see also Leppänen, Zensus, & Diamond 1995). The uniformly weighted restoring (CLEAN) beam obtained was 0.71 × 0.26 mas; the velocity resolution was 0.2 km s−1.

Figure la shows the spatial distribution of the maser spots. Superimposed on the spots are the linear polarization vectors with their lengths proportional to the degrees of polarization. The inset of Figure la is an enlargement of the compact maser concentration near the reference position (0,0) of W51 M. The dotted line in the inset separates blueshifted (west of the dotted line) and redshifted (east of the dotted line) maser spots with respect to the velocity centroid, 61.5 km s−1, of this maser concentration, hereafter called the protostellar cocoon.